critical severity

Potato Late BlightIdentification & Treatment Guide 2026

Phytophthora infestans · Family: Peronosporaceae

Potato late blight (Phytophthora infestans) causes dark water-soaked lesions on leaves and stems, with white sporulation underneath. Tubers develop firm brown rot. Apply preventive mancozeb or chlorothalonil sprays from row closure, destroy infected foliage before harvest, use resistant varieties, and store only disease-free tubers.

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Quick Diagnosis

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Potato Late Blight

Affects: leavesstemstubers
Peak Season: Cool wet weather (15-22°C), rainy seasons
Spread Risk: Fast
Spread By: Airborne sporangia, rain wash from leaves to soil infecting tubers, infected seed tubers

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Symptoms: What does Potato Late Blight look like?

Visual identification guide — tap each stage for details

Water-soaked spots on potato leaf margin from early late blight — Initial water-soaked lesions on leaf tips and edges

Dark brown lesions with white sporulation on potato leaves — Expanding dark lesions with white mold visible on undersides

Cross-section of potato tuber showing reddish-brown late blight rot — Firm granular reddish-brown rot extending into tuber flesh

초기 단계

Water-soaked spots on leaf tips
Irregular, water-soaked, pale green to dark green spots appear on leaf tips and edges, particularly on lower and middle canopy leaves.

중간 단계

Expanding dark lesions with white mold
Lesions expand rapidly, turning dark brown to black. White sporulation visible on leaf undersides during humid conditions. Stems develop dark brown streaks.

후기 단계

Canopy collapse and tuber rot
Entire canopy collapses within 1-2 weeks. Tubers show firm, granular, reddish-brown rot extending inward from the surface. Secondary soft rot bacteria often invade, causing complete breakdown in storage.

Causes: Why is my plant getting Potato Late Blight?

Environmental triggers and disease cycle

Cool wet weather (15-22°C) with extended leaf wetness
Infected seed tubers planted
Volunteer potatoes from previous crop
Proximity to infected tomato crops
Poor drainage and dense canopy

Disease Lifecycle

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Host Plant

Susceptible crop

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Pathogen

Phytophthora

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Conditions

Cool wet weather (15-22°C), rainy seasons

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Infection

Penetrates tissue

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Symptoms

Visible signs

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Spread

Airborne sporangia

Often confused with: Tomato Late Blight, Tomato Early Blight, Downy Mildew. See our comparison guide.

How to Treat Potato Late Blight

Tick each step as you complete it — works offline too

🌿 OrganicSafe for all farms

Organic Treatment Steps

4 steps

Apply copper hydroxide (Kocide 3000) at 1.75 g/L as preventive spray starting at row closure, every 7 days.
Hill potatoes adequately (15-20 cm of soil over tubers) to prevent sporangia washing down to tubers.
Destroy (burn or kill) all foliage 2-3 weeks before harvest to prevent tuber infection during harvest.
Cure harvested tubers at 12-15°C for 2 weeks before moving to cold storage to identify and remove infected ones.

🧪 ChemicalUse protective equipment

Chemical Treatment Steps

4 steps

Apply mancozeb (Dithane M-45) at 2.5 g/L preventively from row closure, every 7-10 days.
Under disease pressure, switch to metalaxyl + mancozeb (Ridomil Gold MZ) at 2.5 g/L for curative + protective action.
Alternate with cymoxanil + mancozeb (Curzate M) and dimethomorph (Acrobat MZ) to manage resistance.
Apply a tuber-protecting fungicide (fluazinam or mancozeb) at final spray before haulm destruction.

Prevention & Seasonal Calendar

Year-round protection plan for your crops

Pre-Planting

Plant only certified, disease-free seed tubers
Choose resistant varieties suited to local races of P. infestans
Hill tubers deeply to protect from rain-splashed sporangia

Mid-Season

Ensure good field drainage — avoid waterlogged soils
Destroy volunteer potatoes that bridge between seasons
Kill foliage 2-3 weeks before harvest to reduce tuber infection

Post-Harvest

Store only fully cured, dry tubers in well-ventilated conditions

🛡️ Resistant Varieties

Sarpo MiraSarpo AxonaBionicaVitabellaToluca

🌼 Companion Planting

These plants can reduce Potato Late Blight pressure when grown nearby:

horseradishgarlic

View full seasonal calendar for your region →

Frequently Asked Questions

Common questions about Potato Late Blight

Can I still harvest potatoes if the foliage has late blight?

Yes, but destroy (mow or chemically desiccate) all foliage at least 2-3 weeks before harvest. This stops sporangia production and allows any existing tuber infections to develop visible symptoms so infected tubers can be sorted out. Wait for dry weather to harvest.

How does late blight get into potato tubers?

Sporangia produced on infected foliage are washed by rain into the soil, where they infect tubers through lenticels or wounds. Tubers can also be infected during harvest if they contact spore-laden foliage or contaminated soil.

Can late blight survive in stored potatoes?

Yes. Infected tubers carry the pathogen through storage. If planted as seed, these tubers produce infected sprouts that become the primary inoculum source for new epidemics. This is why using certified disease-free seed tubers is critical.

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Related Disease

Tomato Late Blight

Phytophthora infestans

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Related Disease

Tomato Early Blight

Alternaria solani

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Affected Plant

Potato

Solanum tuberosum

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Treatment Guide

Full Treatment Protocol

Detailed spray schedule & dosage guide

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